The invention relates to a method for monitoring the exposure of an individual person to toxic gases. The invention also relates to an apparatus for carrying out the monitoring operation.
In many industrial plants, the employees are exposed to most various loadings with toxic gases. The exposure of a person to a toxic gas is physically defined as the time integral of the respective gas concentration measured in the vicinity of the person. The total exposure accumulated during the respective person's working or shift period or a temporal mean value derived from it is of particular interest. Gas detectors for protecting workers, which are worn on the employee's body, differ greatly in design and in performance, the measuring principle being the most decisive factor for the desired information. There are some detectors which trigger an alarm when the maximum allowable concentration of a toxic gas is exceeded in the work place. These detectors usually employ electrochemical sensors which are distinguished by short response times (U.S. Pat. No. 4,141,800). Integrating devices which determine the layer mean value are also known. These devices are based on an air sample collector with which the air prevailing in the work place is drawn continuously through an absorber by means of a small pump. The poisonous substance to be recorded is fixed in the absorber (Offenlegungsschrift No. 26 58 739.3).
Moreover, there is a commercial device which records on a strip of indicator paper the concentration gradient in the form of discoloration of the paper strip. The concentration gradient can then be read by a photometer and recorded. The temporal resolution is determined by the speed of travel of the paper. In a more recent version of this device, the photometer is combined with the detector and triggers an alarm when the mean value exceeds a critical value during the averaging time determined by the speed of travel. An alarm is thus emitted if a specific gas dosage (the product of gas concentration and duration of action) is exceeded during the averaging time predetermined by the device. This alarm function is hereinafter called dosage warning for the sake of simplicity.
According to the prior art, therefore, the various measuring problems, i.e. the detection of the instantaneous value concentration with the emission of an alarm when the critical value is exceeded, the dosage warning, and the detection of the concentration gradient and the layer average value, are solved using various devices. However, it is important to combine all these measures when monitoring a person with respect to exposure to toxic gas, the weighing and the averaging periods having to be determined specifically for each gas. Although with acutely toxic substances there lies the serious consideration of instantaneous warning, an important factor for cumulatively acting substances with an average half-life value, is the adjustment of the correct warning dosage. On the other hand, the warning function is less important in the case of the markedly cumulatively acting substances. In the final analysis, the concentration gradient is detected for a quite different purpose, namely to discover operations which are associated with particularly strong gas subjection. Only a short temporal resolution (a few minutes) is needed for this purpose as, otherwise, the unsystematic short variations would produce a false picture. In the final analysis, the shift mean value (8 hour mean value) can be compared in terms of definition with the MAK (also known as the Threshold Limit Value (TLV) value.
Another complication arises from the fact that quite different measuring methods often have to be adopted for different working substances (gases). A complete program of devices for the protection of workers therefore consists of the most varied components.